In a free society, parties are able to trade things of value to each other. Here, the studio is granting early access to reviewers. These reviewers are competing with each other to see who has the best-written reviews at the earliest point in time, because these are the things that draw readers/viewers which in turn draws advertisers which in turn draws them a paycheck. The value that the studio gets from the deal is free publicity, especially if their movie is good enough to garner positive reviews. Both parties get something out of the deal, so it is in both parties best interest to abide by the terms of the contract. It is neither censorship nor oppression to do so.